An attempt is being made to reduce the weight of an automobile frame through use of a high-strength steel sheet in order to suppress the amount of carbon dioxide exhausted from an automobile. In addition, a high-strength steel sheet as well as a soft steel sheet has been frequently used for automobile frames from the viewpoint of securing the safety of passengers. However, in order to further reduce the weight of an automobile frame in the future, it is necessary to increase the level of operational strength of a high-strength steel sheet compared to the related art.
However, in general, an increase in the strength of a steel sheet results in a decrease in the formability. For example, Non Patent Document 1 discloses that an increase in strength degrades uniform elongation which is important for drawing or stretch forming.
Therefore, in order to use a high-strength steel sheet for underbody components of an automobile frame, components that contribute to absorption of impact energy, and the like, it becomes important to improve local deformability, such as local ductility that contributes to formability, such as burring workability or bending workability.
In contrast to the above, Non Patent Document 2 discloses a method in which uniform elongation is improved by complexing the metallic structure of a steel sheet even when the strength is maintained at the same level.
In addition, Non Patent Document 3 discloses a metallic structure control method in which local deformability represented by bending properties, hole expanding workability, or buffing workability is improved through inclusion control, single structure formation, and, furthermore, a decrease in the hardness difference between structures. The above method is to improve hole expanding properties by forming a single structure through structure control, and, in order to form a single structure, a thermal treatment from an austenite single phase serves as the basis of the manufacturing method as described in Non Patent Document 4.
In addition, Non Patent Document 4 discloses a technique in which metallic structure is controlled through the control of cooling after hot rolling, and precipitates and deformed structures are controlled so as to obtain ferrite and bainite at an appropriate proportion, thereby satisfying both an increase in the strength and securement of ductility.
However, all of the above techniques are a method of improving local deformability through structure control, which is significantly influenced by base structure formation.
Meanwhile, even for improvement of material quality through an increase in the rolling reduction in a continuous hot rolling process, related art exists, which is a so-called grain refinement technique. For example, Non Patent Document 5 describes a technique in which large reduction is carried out at an extremely low temperature range in an austenite range, and non-recrystallized austenite is transformed into ferrite so that the crystal grains of ferrite which is the main phase of the product are refined, and the strength or toughness increases due to the grain refinement. However, Non Patent Document 5 pays no attention to improvement of local deformability which is the object of the present invention.